Electrographotographic charging means



Sept. l5, 1959 A. J. MoNcRlEFF-YATES I' 2,904,431 I EL TR c EC ORAPHOTOGRAPHIC CHARGING BEANS Filed Aug. 26, 1954 /E il United StatesPatent ELECTROGRAPHOTGGRAPHIC CHARGING MEANS Alexander I.Moncriei-Yeates, Granbury, NJ., assignor to Radio Corporation ofAmerica, :a corporation of Delaware Application August 26, F1954, SerialNo. '452,243

2 Claims. k(Cl."'96`1) This invention relates to`improve`d methods -ofand means for producing relectric connection to solid bodies ofsemiconductors and dielectrics, and to improved methods of and meansforelectrostatic printing.

Ordinarily, it is diilicult to provide an electrical connection betweenan electrode `and a body of semiconductive Vor dielectric material. Poorcontact vmay be caused by barrier layers at the surfaces of theelectrode and the body, or byimproper physical contact between the bodyandthe electrode, such as by discontinuous physical contact Vbetween thesurface ofthe body and the surface ofthe electrode.

-Prior methods for providing electric contact to such bodies include:ionizing the Agas inthe space Vbetween the electrode and the surfaceofthe semiconductive or dielectric body which requires either highvoltages or high electric currents and is ineflicient; pyroelectricmethods 'which are inconvenient; iand triboelectric methods which aredifficult to-control.

Another method for providing-anfele'ctric contact is to apply anelectrode to the stu'face ofthe body-of semiconductive Sor dielectricmaterial and `then 'to Vapply a known voltage with respect to the bodyto thefelectrode. While small amounts ofcharge are transferred `Vibythis method, the transfer isunequal'across ithe electrode surface, evenwhen highvoltages are applied.

It has 'been lproposed to sandwich a 'thin sheet of pliablesemiconducting*material such'as'rubbenmaper or plastic sheets betweenthe electrode andthe bo'clyin order to improve the connection. Whilethis method is an improvement over the structure `having no `-suchsemiconducting sheet, still the required yvoltages 'are `high andthetransfer is uneven.

Itfhas been further proposedto improve the connection between anelectrode and abody ofserniconductingor dielectric material byinterposing therebetween a thin layer of yan electrically-conducting-1iquid. 'While this method :provides irnproved' electric contactsbetween the electrode `and the-body, it has "the disadvantage t that theliquid must be-insulated from adjacenttconductors. rFurthermore, theliquid layeriseonductive laterally along its length as well astransversely through its thickness. Where it is desired to pass apattern of electric-charge or .current, -this method dissipates .thepattern laterally.

It-isan-object of this inventionto provide improved electricalconnections to ibodies yof vsemiconductors Yand v dielectrics.

or dielectric material which comprises closely spacingthe surface of anelectrode from the surface of the 'body to which connection is to bemade, with a ilm forming liquid such as lpropyl alcohol which iselectrically-insulating in bulk. When a voltage with respect to the bodyis applied to the electrode, the electric field across the liquidunexpectedly causes the liquid to behave as a conductor transverselythroughthe layer while continuing'to behave as an insulatorin thelateral direction. By semiconductive or dielectric material is meant 1a`material having a resistivity greater than 106 ohm-cm.

The invention includes a method of Velectrically charging the surface ofa .body of semiconductive or dielectric material which ineludesthe abovedescribed step *for providing an electrical connection in combinationwith the subsequent steps of applyinga voltage withrespe'ct to the bodyto the electrode, removing said electrode from said body, removingthevoltage from said lectrode and then removing vthe liquid'remaning on thesurface of said body.

The invention includes improved vmethods andmeans for electrostaticprinting including the kimproved lelectrical connections and theimproved methods of charging.

The invention will be described in greater detail vby reference totheaccompanying drawing inwhich:

Figure 1 is a sectional, elevational view of animproved means for makingan electrical connection betweena conductor and a dielectric.

"Figure 2 is a sectional, elevational view'of 'animproved means formaking an electrical connection between a conductor and a semiconductor,and

Figure 3 is a sectional, elevational viewof an -improved means forymaking an electrical connectionbetween a semiconductor and adielectric.

Similar reference ycharacters are applied to similar elements throughoutthe drawing.

YExample .1

-.Referring to Figure l, Janimproved velectrical .connection means"between a vconductor and a dielectric comprises Va sheet 21 ofdielectric material-sandwiched fbetween an upper .electrode 41 andalower electrode 43. The-dielectric 'material y21 may 'be for example .a=sheet of polystyrene 0.0005 to0.0l0finchthi'ck preferably about 0.003inchfthick. The electrodesfi-land 43may'lbeofiany conducting material,for example, brass.

In order to improve the electrical connection between theupper electrode41 and'thedielectric sheet`21, athin liquid layer 31 of propylalcohol'isinterposed'therebetween, The -layer'rnay be about 0.00002to10.003 yinch thick,.preferably0:0001 inch thick.

A uniform electric charge is .formed on the upper surface of the4dielectric sheet "21 by applying between about '100 :and 2000 -volts tole'a'ds 45 fatta'ched to fthe electrodes 41 and 43. Forthe preferredlthicknesses of elements,.about v200'volts is preferred. `Withthickerelements thehigher voltages areipreferred.

In order'to producea similar charge on'the upper surface of thedielectric sheet 21 Without'the liquidlayer 31, tbetween about 1000 'and30,000 volts are required. For vthe ,preferred thicknesses of felements,`about 7000 volts .arerequired `Furthermore;'the resulting charge on thesurfacezof the dielectric sheet isnot asuniformas'the charge formed vbythemethod'of `Example l. IThis lack of uniformity VVmay be duetosurfaceirregularities of th'e surfaces ofthedielectricsheet or Vtheelectrode or both resulting rin discontinuous physical contacttherebetween.

If a thin sheet of a pliable, semiconductor, suchas paper about 0.002inch thick is substituted furthe-liquid layer 31, about -1000to 7000volts is required-to charge the surfaceof the dielectricsheet '21. Forthepreferred thicknessabout'5000\voltsi -isfrequired The charged surfacemay be uncovered by removing the electrodes 41 and 43 and evaporatingthe propyl alcohol remaining on the surface of the dielectric sheet 21.The charged surface may then be used for any desired purpose.

Example 2 The electrical connecting means of the invention may be usedin an electrostatic printing process to provide an improved method ofblanket charging. An electrostatic printing process is that type ofprocess for producing a visible image, reproduction or copy whichincludes, as an intermediate step, converting a light image or electricsignal to an electrostatic charge pattern on an electricallyinsulatingbase. The process may include the conversion of the charge pattern intoa visible image which may be a substantial reproduction of an originalexcept that it may be different in size. A detailed description of thesteps of such a process may be found in U.S Patent 2,297,691, issuedOctober 6, 1942, to C. F. Carlson.

In a typical electrostatic printing process, a layer of photoconductivematerial disposed upon an electricallyconductive backing is providedwith a blanket electrostatic charge. A light image is then focused uponthe photoconductive layer. 'Ihe electrical resistance of the illuminatedareas of the photoconductive layer is reduced, allowing an electrostaticimage corresponding to the light image to form on the photoconductivelayer.

The blanket electrostatic charge may be provided triboelectrically,pyroelectrically, by irradiation with electrons or ions or by physicalcontact with a charged electrode. Each of these processes hasdisadvantages as described above.

Referring to Figure 2, a layer 23 of a photoconductive material such asselenium supported upan an electrically conducting backing 25, such aspaper, is sandwiched between an upper electrode 41a and a lowerelectrode 43a. The backing 25 rests upon the lower electrode 43a. Theelectrodes may be of any conductive material such as brass.

In order to improve the electrical connection between the upperelectrode 41a and the photoconductive layer 23, a thin liquid layer 31aof gasoline is interposed therebetween. The liquid layer may be about.0001 to .002 inch thick but is preferably .0005 inch thick.

A uniform electric charge is formed on the upper surface of thephotoconductive layer 23 by applying between 100 and 500 voltspreferably about 300 volts to leads 45a attached to the electrodes 41aand 43a. The structure is then disassembled and the gasoline remainingon the surface of the photoconductive layer 23 is evaporated leaving adry surface having a uniform electrostatic charge thereon. The surfacemay then be exposed photographically and developed by conventionalmeans.

Example 3 The electrical connecting means of the invention may be usedto provide an improved electrostatic printing process. In anothertypical electrostatic printing process, an electric iield is appliedthrough a layer of photoconductive insulating material; for example, bycontacting a rst electrode with one side of the photoconductive layer,spacing a second electrode from the other side of said photoconductivelayer and applying voltage between said electrodes. When a light imageis focused on the photoconductive layer, the electric resistance of theilluminated areas of the photoconductive layer is reduced allowing anelectrostatic image to form in the photoconductive layer. Theelectrostatic image is conducted to a dielectric surface and thendeveloped in the usual way.

Referring to Figure 3, a sheet 27 of dielectric material such as apolystyrene resin is sandwiched between a lower electrode 43h, forexample, brass, and an upper transparent electrode 41b having a layer 29of photoconductive material on the inner surface thereof. Thetransparent electrode may comprise a sheet of glass 39 having a turfacetreated with tin chloride at about 400 C. to produce a conductive layer41h thereon. The photoconductive layer 29 may be cadmium sulphide bondedin 20% by weight of a silicone resin, but is preferably photoconductiveselenium about .001 inch thick.

In order to improve the electrical connection between thephotoconductive layer 29 and the dielectric sheet 27, a thin liquid film3117 of cyclohexanol is interposed therebetween. The layer may be about.0005 to .003 inch thick but preferably about .002 inch thick.

A negative voltage of about 400 volts is applied to the upper electrode41b with respect to the lower electrode 43b lby leads 45b attachedthereto. About 2000 volts is required to obtain the same result if theliquid film of the invention is not used. Simultaneously, a light imageis focused upon the photoconductive layer 29, for example with a lens71. The electrical resistance in the illuminated areas of thephotoconductive layer is reduced, allowing an electric charge to passtherethrough and to deposit in the dielectric sheet 27 thereby formingan electrostatic image thereon corresponding to the light image focusedon the photoconductive layer.

The voltage and the light image are removed and the apparatusdisassembled. The liquid remaining on the dielectric sheet 27 isevaporated and the electrostatic image thereon is developed byconventional methods. The use of liquid film 31h not only improves theelectrical connection between the photoconductive layer 29 and thedielectric sheet 27, but also the electric charge pattern is transferredwith a minimum dissipation of the pattern laterally along the contactsurface.

Poor electrical connection is ordinarily encountered where the contactis between conductor and semiconductor, conductor and dielectric,semiconductor and semiconductor and semiconductor and dielectric. Ineach of the above described examples electrical connection is improvedby interposing a thin lm of a liquid which is ordinarilyelectrically-insulating in bulk. When such a material is reduced to athin ilm or layer, it behaves as if it is a conductor or asemiconductor. Thus, the film will conduct electric charges through thelm but remains electrically-insulating laterally along the length of thecontact. The practice of the invention affords the equivalent of optimumphysical contact between the bodies and a minimum in electric barriersto the flow of electricity.

The liquid used to produce the improved electrical connection of theinvention is preferably a film-forming material so as to provide acontinuous layer over the surface. The liquid has preferably, a bulkyresistivity greater than 109 ohm-cm. so as to minimize the lateral ilowof electric charge along the length of the contact. 'I'he liquid has,preferably, a dielectric `constant greater than 5 so as to obtain aminimum drop in voltage across the liquid layer. Ihe liquid should be ofa material that is substantially non-ionizing in order to avoidhydrolysis. The liquid should be chemically stable for the duration ofits use and should not attack the electrode or the contacted body.

Examples of other suitable liquids are propyl alcohol, gasoline,cyclohexanol, cyclohexane, amylacetate propylene glycol, acetone,polystyrene dissolved in acetone, polyvinyl chloride in diethylenechloride, water soluble cellulose, distilled water and kerosene.

The electrode is preferably a good conductor of electricity but may be abody having a resistivity as high as 1010 ohm-cm. The body to whichconnection is made may be a dielectric or semi-conductor and preferablyhas a resistivity greater than l06 ohm-cm.

There have been described improved electrical connection means tosemiconductors and dielectrics. These connection means may be used toprovide a uniform electrostatic charge on the surface of a body ofsemiconduc- 5 tive or dielectric material, and to provide improvedmethods and means of electrostatic printing.

What is claimed is:

1. A method of electrostatic printing comprising the steps -of coatingthe surface of a photoconductive insulating material on a conductivebacking member with a thin lm of a liquid having a bulk resistivity ofat least 109 ohm-centimeters, closely spacing an electrode which isconductive in bulk from said photoconductive insulating material and inarea contact with the surface of said film of liquid, establishing anelectric potential between said electrode and said backing member of amagnitude sui cient to produce an electrostatic charge at the surface ofsaid photoconductive insulating material, removing said electrode fromsaid closely spaced relationship, removing the liquid remaining on thesurface of said photoconductive insulating material, photographicallyexposing the surface of said photoconductive insulating material therebyproducing an electrostatic latent image on the surface of saidphotoconductive insulating material and then developing saidelectrostatic latent image with an electrostatically attractablesubstance.

2. A method of electrostatic printing comprising the steps of coatingthe surface of a photoconductive insulating material on a conductivebacking member with a thin lm of a liquid having a bulk resistivity ofat least 109 ohm-centimeters, closely spacing an electrode which isconductive in bulk from said photoconductive insulating material and inarea contact with the surface of said lm of liquid, establishing anelectric potential between said electrode and said backing member of amagnitude sucient to produce an electrostatic charge at the surface ofsaid photoconductive insulating material, photographieally exposing thesurface of said photoconductive insulating material thereby producing anelectrostatic latent image on the surface of said photoconductinginsulating material, then removing said electrode from said closelyspaced relationship and removing the liquid remaining on the surface ofsaid photoconductive insulating material and then developing saidelectrostatic latent image with au electrostatically attractablesubstance.

References Cited in the ile of this patent UNITED STATES PATENTS 779,190Thomson Jan. 3, 1905 1,977,263 Meissner Apr. 9, 1935 2,181,946Lindenblad Dec. 5, 1939 2,277,013 Carlson Mar. 17, 1942 2,297,691Carlson Oct. 6, 1942 2,438,959 Bartelheim et al. Apr. 6, 1948 2,464,405Knauf Mar. 15, 1949 2,538,554 Cherry Ian. 16, 1951 2,618,551 Walkup Nov.18, 1952 2,689,830 Sadowski et al Sept. 21, 1954 2,692,948 Lion Oct. 26,1954 2,740,184 Thomas Apr. 3, 1956 2,758,939 Sugarman Aug. 14, 19562,774,921 Walkup Dec. 18, 1956 2,825,814 Walkup Mar. 4, 1958 OTHERREFERENCES Wainer: Photographic Engineering, vol. 3, No. 1, pages 12-22(1952). (Copy in Scientiiic Library.)

1. A METHOD OF ELECTROSTATIC PRINTING COMPRISING THE STEPS OF COATINGTHE SURFACE OF A PHOTOCONDUCTIVE INSULATING MATERIAL ON A CONDUCTIVEBACKING MEMBER WITH A THIN FILM OF A LIQUID HAVING A BULK RESISTIVITY OFAT LEAST 10**9 OHM-CENTIMETERS, CLOSELY SPACING AN ELECTRODE WHICH ISCONDUCTIVE IN BULK FROM SAID PHOTOCONDUCTIVE INSULTATING MATERIAL AND INAREA CONTACT WITH THE SURFACE OF SAID FILM OF LIQUID, ESTABLISHING ANELECTRIC POTENTIAL BETWEEN SAID ELECTRODE AND SAID BACKING MEMBER OF AMAGNITUDE SUFFICIENT TO PRODUCE AN ELECTROSTATIC CHARGE AT THE SURFACEOF SAID PHOTCONDUCTIVE INSULATING MATERIAL, REMOVING SAID ELECTRODE FROMSAID CLOSELY SPACED RELATIONSHIP, REMOVING THE LIQUID REMAINING ON THESURFACE OF SAID PHOTOCONDUCTIVE INSULATING MATERIAL, PHOTOGRAAPHICALLYEXPOSING THE SURFACE OF SAID PHOTOCONDUCTIVE INSULATING MATERIAL THEREBYPRODUCING AN ELECTROSTATIC LATENT IMAGE ON THE SURFACE OF SAIDPHOTOCONDUCTIVE INSULATING MATERIAL AND THEN DEVELOPING SAIDELECTROSTATIC LATENT IMAGE WITH AN ELECTROSTATICALLY ATTRAACTABLESUBSTANCE.